There are a lot of variables with multi's Dave but the biggest one is prop diameter.

The small diameter props often needed on multi setups - especially scale military setups - result in less over all thrust than a similarly powered single prop setup.

Still, the old watts/pound rules still apply pretty much. if you can get 100 watts/pound you should have pretty good flight performance. 75 to 80 would probably be more than enough to fly scale with some margin and 150-200 would be a rocket

From a few experiments flying control liners with nitro motors, I know that 2 motors on the same plane give you much more performance than one motor with the total displacement of the 2.

I have 4 matched 3536 1000KV brushless motors. Each can put out a little over 450 watts.

I want to build a couple of twins, or a B-17.

How heavy could a plane be in each case, with good flight performance?

I found a program for a Quadcopter, but not a multi plane.

Check out www.motocalc.com,free for 30 days, then $39. This program allows entering in wingspan, area,weight, prop and one or more motors up front. The program then spits out all sorts of graphs, charts and the like. As for me, I just check out the predicted rate of climb, watts/volts/amps, watts per pound and the opinions tab. This program also has a number of other bits of info to check over.